Wow. I didn't know you could upload a list of your customer's email addresses to Facebook and have it build an audience of similar users. This seems highly unethical -- who knows what FB will do with your customer's email addresses.
Edit: I also wonder what would happen if I uploaded just my own email address? Would it find people very similar to me?
You can't upload just one email address (they changed the minimum number a long time ago for privacy reasons). There is a minimum number to create a custom audience of email addresses and then you create a lookalike audience that is 1MM+ people. I don't see how that's a violation of privacy.
It's probably the most effective thing Facebook has done for targeting as I use almost exclusively custom audience and lookalike targeting.
This is now a universal feature offered by Google, Bing, Yahoo Gemini, Twitter, Linkedin and I'm sure others. It is no different than cookies for retargeting. leveraging first party data is by far the biggest opportunity in advertising and this trend is here to stay.
No one is using that data except the advertiser who uploads it.
Edit: I also wonder what would happen if I uploaded just my own email address? Would it find people very similar to me?